<p>In this paper, the effects of La treatment and Al–La composite treatment on sulfur-containing peritectic steel were compared and studied. The evolution mechanism of inclusion characteristics and the change law of microstructure in sulfur-containing peritectic steel after La treatment and Al–La composite treatment were revealed. The results show that La treatment can preferentially combine with a large amount of S in sulfur-containing peritectic steel, thus changing MnS into La₂O₂S and LaAlO₃–La₂O₂S inclusions. The Al–La composite treatment increases the Al content, thereby changing MnS into LaAlO₃ and La₂S₃ inclusions. After La treatment and Al–La composite treatment, the inclusions were purified, and their number density and average size decreased. The pinning effect of La₂O₂S, LaAlO₃ and La₂S₃ inclusions refined grains, and the austenite grain sizes decreased from 421.4 to 192.7 and 148.4 <i>μ</i>m, respectively. The Mn-depleted zone (MDZ) near MnS in austenite grain boundary of untreated sulfur-containing peritectic steel is beneficial to grain boundary ferrite (GBF) nucleation, while the La₂O₂S and La₂S₃ formed after La treatment and Al–La composite treatment are beneficial to acicular ferrite (AF) nucleation. Therefore, the AF content increased by 33.8 and 18.9 pct, and the GBF content decreased by 21.0 and 15.5 pct, respectively.</p>